Characteristics of two-component two-phase flow and heat transfer in a flat microchannel

Experiments are reported on heat transfer and flow characteristics of two-component, two-phase flow in a flat microchannel with cross-section 0.1 × 30 mm. The channel is heated at nearly constant wall temperature on one side for a length of 30 mm. Water and FC-72 ® are the working fluids, and local temperature data and high-speed video of the two-phase flows are reported. Total mass flux ranges from 40 and 467 kg/m²s, with mass fraction of FC-72 between 0.04 and 0.69. All flows are sub-cooled up to 32°C below the saturation temperature of FC-72 and wall heat flux is controlled so that boiling of the FC-72 component occurs. The rate of FC-72 flow through the channel is unsteady due to conditions upstream of the microchannel, which cause temperature fluctuations in the channel. The components remain distinct within the microchannel, as predicted by the low Weber numbers of the flows, and the fraction of the channel cross-section occupied by FC-72 varies inversely with water flow rate. At high water flow rates waviness develops in the interface between the FC-72 and water streams and some droplets of FC-72 leave the main body of the flow. Under some conditions pressure drop oscillation in the boiling FC-72 stream is observed. Estimates of the average heat transfer coefficient are obtained based on average wall temperatures. Large mass fractions of FC-72 reduce overall heat transfer coefficients, indicating that only small mass fractions of FC-72 are needed to produce heat transfer enhancement. Upstream mixing of the two components is seen as key step in fluid management to assure improved heat transfer.